This invention relates to electrode compositions useful in secondary lithium batteries.
Two classes of materials have been proposed as anodes for secondary lithium batteries. One class includes materials such as graphite and carbon which are capable of intercalating lithium. While the intercalation anodes generally exhibit good cycle life and coulombic efficiency, their capacity is relatively low. A second class includes metals that alloy with lithium metal. Although these alloy-type anodes generally exhibit higher capacities relative to intercalation-type anodes, they suffer from relatively poor cycle life and coulombic efficiency.
The invention provides electrode compositions suitable for use in secondary lithium batteries in which the electrode compositions have high initial capacities that are retained even after repeated cycling. The electrode compositions also exhibit high coulombic efficiencies. The electrode compositions, and batteries incorporating these compositions, are also readily manufactured.
To achieve these objectives, the invention features, in a first aspect, an electrode composition that includes (a) an electrochemically active metal element and (b) a non-electrochemically active metal element. When incorporated in a lithium battery and cycled through one full charge-discharge cycle, the composition includes crystalline regions having at least one dimension that is no greater than about 500 angstroms (preferably no greater than about 100 angstroms, more preferably no greater than about 50 angstroms) and that does not substantially increase after a total of at least 10 cycles (preferably at least 100 cycles, more preferably at least 1000 cycles).
The crystalline regions may be present prior to cycling or may appear only after one full charge-discharge cycle. In the former case, the regions persist after one full charge-discharge cycle.
An xe2x80x9celectrochemically active metal elementxe2x80x9d is an element that reacts with lithium under conditions typically encountered during charging and discharging in a lithium battery. A xe2x80x9cnon-electrochemically active metal elementxe2x80x9d is an element that does not react with lithium under those conditions. In both cases, the metal element may be found in the electrode composition in the form of a metal containing only the element itself (i.e., an elemental metal) or a compound containing the element in combination with one or more elements which may or may not be metal elements. An example of the latter is an intermetallic compound containing the metal element in combination with one or more metal elements. Prior to cycling, however, the electrochemically active metal element is in the form of an intermetallic compound or an elemental metal.
Where the electrochemically active metal element is part of a compound, the compound itself need not be electrochemically active, although it may be. Similarly, where the non-electrochemically active metal element is part of a compound, the compound itself need not be non-electrochemically active, although it may be.
An example of a preferred electrochemically active metal element is tin. Examples of preferred non-electrochemically active metal elements include molybdenum, niobium, tungsten, tantalum, iron, copper, and combinations thereof. Particularly preferred electrode compositions are those in which (a) the electrochemically active metal element is tin and the non-electrochemically active metal element is molybdenum; (b) the electrochemically active metal element is tin and the non-electrochemically active metal element is iron; (c) the electrochemically active metal element is tin and the non-electrochemically active metal element is niobium; (d) the electrochemically active metal element is tin and the non-electrochemically active metal element is tungsten; and (e) the electrochemically active metal element is tin and the non-electrochemically active metal element is tantalum.
The crystalline regions are characterized by a discernible x-ray diffraction pattern characteristic of a crystalline material. In terms of chemical composition, at least one of the crystalline regions preferably includes the electrochemically active metal element and at least another of the crystalline regions includes the non-electrochemically active metal element.
The crystalline regions are preferably separated by regions comprising the electrochemically active metal element and the non-electrochemically active metal element in which the relative proportions of the these elements vary throughout the thickness direction of the composition. For cases in which the electrode composition is in the form of a thin film, the xe2x80x9cthickness direction of the compositionxe2x80x9d refers to the direction perpendicular to the substrate on which the film is deposited. Where the electrode composition is in the form of a powder representing a collection of individual particles, the xe2x80x9cthickness direction of the compositionxe2x80x9d refers to the thickness direction of an individual particle.
The regions separating the crystalline regions do not exhibit an electron diffraction pattern characteristic of a crystalline material. They may be present prior to cycling, after cycling, or both before and after cycling.
When the electrode composition is incorporated in a lithium battery and cycled to realize about 100 mAh/g of electrode composition, the electrode composition preferably exhibits a coulombic efficiency of at least about 99.0% (more preferably at least about 99.8%, even more preferably about 99.9%) after 100 full discharge cycles. The electrode composition may be provided in the form of a thin film or a powder.
In a second aspect, the invention features an electrode composition that includes (a) an electrochemically active metal element and (b) a non-electrochemically active metal element. When incorporated in a lithium battery and cycled through one full charge-discharge cycle, the composition includes crystalline regions. In addition, when incorporated in a lithium battery and cycled to realize about 100 mAh/g of electrode composition, the electrode composition exhibits a coulombic efficiency of at least about 99.0% (preferably at least about 99.8%, more preferably at least about 99.9%) after 100 full discharge cycles.
The crystalline regions may be present prior to cycling or may appear only after one full charge-discharge cycle. In the former case, the regions persist after one full charge-discharge cycle.
The electrochemically active metal element and non-electrochemically metal element have the definitions described above. In both cases, the metal element may be found in the electrode composition in the form of a metal containing only the element itself (i.e., an elemental metal) or a compound containing the element in combination with one or more elements which may or may not be metal elements. An example of the latter is an intermetallic compound containing the metal element in combination with one or more metal elements. Prior to cycling, however, the electrochemically active metal element is in the form of an intermetallic compound or an elemental metal.
The electrode composition may be provided in the form of a thin film or a powder. An example of a preferred electrochemically active metal element is tin. Examples of preferred non-electrochemically active metal elements include molybdenum, niobium, tungsten, tantalum, iron, copper, and combinations thereof. Particularly preferred electrode compositions are those in which (a) the electrochemically active metal element is tin and the non-electrochemically active metal element is molybdenum; (b) the electrochemically active metal element is tin and the non-electrochemically active metal element is iron; (c) the electrochemically active metal element is tin and the non-electrochemically active metal element is niobium; (d) the electrochemically active metal element is tin and the non-electrochemically active metal element is tungsten; and (e) the electrochemically active metal element is tin and the non-electrochemically active metal element is tantalum.
The crystalline regions are characterized by a discernible x-ray diffraction pattern. In terms of chemical composition, at least one of the crystalline regions preferably includes the electrochemically active metal element and at least another of the crystalline regions includes the non-electrochemically active metal element.
The crystalline regions are preferably separated by regions comprising the electrochemically active metal element and the non-electrochemically active metal element in which the relative proportions of the these elements vary throughout the thickness direction of the composition (as defined, above). These regions separating the crystalline regions exhibit no discernible electron diffraction pattern characteristic of a crystalline material. They may be present prior to cycling, after cycling, or both before and after cycling.
In a third aspect, the invention features a method of preparing an electrode composition that includes combining (a) a source comprising an electrochemically active metal element and (b) a source comprising a non-electrochemically active metal element to form an electrode composition characterized in that: (i) when incorporated in a lithium battery and cycled through one full charge-discharge cycle, the electrode composition includes crystalline regions and (ii) when incorporated in a lithium battery and cycled to realize about 100 mAh/g of the composition, the electrode composition exhibits a coulombic efficiency of at least about 99.0% after 100 full discharge cycles.
In one preferred embodiment, the source of the electrochemically active metal element and the source of the non-electrochemically active element are sequentially sputter-deposited onto a substrate. In another preferred embodiment, the two sources are combined by ball milling.
The above-described electrode compositions may be combined with a counterelectrode and an electrolyte separating the electrode and counterelectrode to form a lithium battery.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.